Recently, wide spread of the Internet increases the demand for a smaller-size optical system. One of the solutions thereof now investigated is to integrate together optical-waveguide devices each including an optical waveguide. The techniques to be employed for integration of these optical devices include monolithic integration and hybrid integration. The monolithic integration is the ultimate technique for achieving a smaller size; however, it is difficult to optimize the functions of the individual optical devices because of the restriction on the fabrication process, the non-negligible influence of the reflected and returned light in the structure of integrated laser devices, and so on. In addition, due to a larger size of the optical device and a complicated fabrication process, there is a concerned possibility of a lower product yield.
On the other hand, the hybrid integrated device attracts a larger attention due to the easy optimization of the functions of the individual optical devices, easy design change, capability of integration of optical devices made from different materials and wider range of application thereof, although the hybrid integrated device necessitates a larger number of parts, such as optical lenses, required for optical coupling.
In the hybrid integrated device, it is especially important to position-align the optical-waveguide devices with each other. The conventional techniques for positioning of the devices in the vertical direction include one wherein a sub-mount for mounting thereon an optical-waveguide device is aligned in advance with another optical-waveguide device in the vertical direction, and one wherein a spacer is used for adjustment if the vertical adjustment is needed. The former technique is described in Patent Publication JP-2004-289011A, for example.
It is to be noted in the semiconductor waveguide that there arises a significant range of variation as to the position of the optical waveguide in the vertical direction, i.e., parallel to the stacking direction of semiconductor films, because the semiconductor substrate is subjected to polishing in the fabrication process. In addition, since the dimension of the waveguide is as small as in the range of several micrometers, and restricts the coupling tolerance between the optical-waveguide devices, a mere fine adjustment of the coupling lens may not well improve the coupling efficiency. Thus, a hybrid optical integrated device has been desired which allows an easy position alignment.
Patent Publication JP-2001-59925A describes an optical module which affords easy position alignment upon optically coupling together an optical-waveguide device including an optical waveguide and an optical element such as an optical lens. The optical module described in JP-2001-59925A employs the structure in which a holder mounting thereon optical elements including an optical isolator and an optical lens is position-aligned with an optical-waveguide device provided in the optical module in the state where the holder is supported on a holder support, and thereafter the holder is fixed onto the holder support that is fixed onto a base table in advance.
In the structure described in JP-2001-59925A, there is an advantage in that the position alignment between the optical element and the optical-waveguide device is achieved with ease. However, this publication is silent as to the structure wherein a plurality of optical-waveguide devices, if provided therein, are position-aligned with each other via an optical element, although the publication describes the position alignment between the optical element and the optical-waveguide device.